a) Field of the Invention
The present invention relates to a method for forming a Fresnel lens destined for use as condensing lens in an automotive lighting, etc. and a Fresnel lens formed by the same method, and more particularly to a method for forming a Fresnel lens having excellent optical properties easily and with a high precision and which can minimize the defect on the lens surface caused by a flow of the glass material in the process of pressing.
b) Description of the Prior Art
Fresnel-type prism lens is generally formed from a synthetic resin or glass. In case a Fresnel-type prism lens is formed from a synthetic resin by injection molding, the process is relatively easy and the resulted lens has a smooth surface. However, the forming of such lens from a glass by pressing is disadvantageous in that the flow of a molten glass near the glass material in the process of pressing results in a surface defect which deteriorates the surface smoothness and optical properties of the lens and also that the difference in temperature distribution of the molten glass between near the the surface and the inside thereof becomes considerably larger as the time passes. So it is desired to reduce the forming time and improve the yield so that the forming is completed at a temperature near the glass transition point or transformation temperature.
FIGS. 1 (A) to (C) show a conventional method, namely, a gob pressing method, for forming a Fresnel-type prism lens from a molten glass by pressing. As shown in FIG. 1 (A), a glass material 1 is put on a stationary mold 2 having a nearly flat surface. A female mold 3 having an irregularity of a configuration derived from inversion of the configuration of each element of a prism to be formed is installed on a press (not shown in the drawing). As shown in FIG. 1 (B), the female mold 3 is moved down in the direction of arrow X toward the glass material 1 on the stationary mold 2. The glass material 1 is pressed by the female mold 3 into a partially fabricated item 4. This partially fabricated item 4 is further pressed and finally formed into a Fresnel-type prism lens 8 having prism elements 6 of predetermined shapes as shown in FIG. 1 (C).
In the process of pressing in which a gob-like glass material 1 is pressed into a partially fabricated item 4 (shown in FIG. 1 (B), as the gob-like glass material 1 is pressed outwardly at a temperature near the yield point at which a material reaches its maximum coefficient of expansion, shows a relatively low plasticity and starts shrinking, so that the molten glass especially near the surface of the gob-like glass material 1 flows in the direction of arrow a along the convex and concave surfaces forming the female mold 3, that is, the molten glass flows from a concave surface toward an outer concave surface beyond a convex surface between these concave surfaces. In the process shown in FIG. 1 (C), other molten glass than near the surface of the gob-like glass material 1 is also subject to outward pressing. Thus, the surface of the prism lens 8 finally formed incurs surface defects like wrinkle, etc. as the case may be. The prism lens 8 thus formed has a poor surface smoothness and also poor optical properties.
To overcome the above-mentioned drawbacks of the conventional method of gob pressing, methods for forming a glass article from a molten sheet glass have been proposed (one example is disclosed in the U.S. Pat. No. 4,361,429), which, however, cannot provide any surface configuration of a high precision such as a Fresnel-type prism lens having a plurality of aspherical surfaces for predetermined optical properties.
Generally, the aspherical geometric shape forming each of the above-mentioned prism elements is given as a special solution of a certain partial differential equation. However, it does not suffice for a practical aspherical geometric shape to provide such special solution but the shape is determined taking in consideration of the relation among the other design requirements including the thickness of a substrate on which the aspherical surface of each prism element is formed (substrate thickness), maximum thickness including the aspherical surface of each prism element and maximum distance between prism elements (pitch). The geometrical shapes thus determined of aspherical surfaces forming the prism elements are all different from one another and the pitch is not constant. Therefore, the volumes of the prism elements are all different from one another as well. Hence, the above-mentioned problem that the surface defects are caused by the flow of the molten glass near the surface of the glass material from a cavity corresponding to each prism element into a cavity corresponding to an adjacent prism element still remains unsolved.